Large bowel adenomatous polyps present a unique opportunity to conduct an intervention trial because of the high prevalence rate in the general population, the high polyp recurrence rate in those who have undergone polypectomy, and the link between polyps and cancer. It is generally accepted that large bowel adenomas are a requisite precursor lesion for most large bowel cancers. Given the strong evidence for the polyp-cancer sequence, an intervention that reduces the recurrence of large bowel polyps would have a strong likelihood of reducing the incidence of large bowel cancer. The major objective of this study is to determine whether an experimental large bowel cancer "risk reduction" diet (low fat, high fiber, vegetable- and fruit-enriched) will decrease the recurrence rate of large bowel adenomatous polyps. This will be a multi-center randomized controlled trial involving 2,000 men and women. Study participants will be randomized into either the experimental diet group or a control group (usual diet). Recruitment will take up to two years, and the follow-up time from randomization is four years. The study has two secondary objectives: 1) to investigate the relation between the dietary intervention and several putative intermediate endpoints in large bowel carcinogenesis, and 2) to evaluate the correspondence between these intermediate endpoints and subsequent neoplasia (adenoma formation). Particular intermediate endpoints of interest include mucosal cell proliferation, ornithine decarboxylase, and mucins.